Accidentally messed up the geometry on one speaker. Can I fix it somehow? I already can't undo it.

Hi Valentine VValentineSokil ,

It looks like there is more wrong with your model, so it's really best to start over (believe me, it's counterintuitive, but absolutely worth it!).

But what you can do for the speaker, is make a connection (yellow line) between two Vertices with CTRL+J and then Delete all Vertices of the left Speaker nad Duplicate the right Speaker and move it in place and Fill the hole with two Faces, similar to the right side.

Yeah doubling agreeing. Starting over is great and you can always Shift + D to duplicate a selection. Things always have fixes, but as you model more and more and you waste too much time trying to fix something, you'll see it's quicker and better to start over.